A backlight source or a backlight module of a display device usually employs a light guide plate for providing a planar light source. Size of the light guide plate needs to be fit for the size of the display device. However, it is difficult for a large-sized light guide plate to emit enough uniform light beams. Moreover, for an edge-emitted light guide plate, an optical length of light emitted into the large-sized light guide plate obviously increases, which results in loss of light energy. In addition, the fabrication and transporting difficulty also increases with the increase of the size of the light guide plate.